1. Field of the Invention
The invention relates to peptide mimetics, which are chemical structures which serve as appropriate substitutes for peptides in interactions with receptors and enzymes. The invention also relates to means for analyzing specific interactions between peptides and enzymes or receptors, as well as for developing new therapeutic agents, through the use of peptide mimetics.
2. Summary of the Related Art
Peptides and proteins play critical roles in the regulation of all biological processes. Peptides, for example, play a regulatory role as hormones and inhibitors, and are also involved in immunological recognition. The significant biological role of peptides makes important the understanding of the interactions between peptides and their receptors or enzymes to which they bind.
The determination of the receptor-bound conformation of a peptide is invaluable for the rational design of peptide analogues. However, Marshall et al., Ann. Rep. Meal. Chem. 13: 227-238 (1978), discloses that peptides are characteristically highly flexible molecules, the structures of which are strongly influenced by the environment in which they reside. Thus peptides are not generally useful for determining their receptor-bound conformation.
As no approach is available to predict a priori which new ligand-receptor interactions will lead to antagonists and which will lead to agonists of greater or less potency, it is necessary to perform classical structure-function studies in a systematic way to provide information about the specific amino acid residues and functional groups in a peptide that are important to biological activity. Studies of this nature can utilize conformationally constrained peptide mimetics. For example, Hruby, Trends Pharmacol. Sci. 8: 336-339 (1987), suggests that conformational constraints can provide information about the different requirements that a receptor has for a ligand to be an agonist or antagonist.
Generally, peptide mimetics can be defined as structures which serve as appropriate substitutes for peptides in interactions with receptors and enzymes. The development of rational approaches for discovering peptide mimetics is a major goal of medicinal chemistry. Such development has been attempted both by empirical screening approaches and by specific synthetic design.
Screening of pure chemical entities has been of quite limited utility for discovering peptide mimetics. However, Chipkin et al., Ann. Rep. Meal. Chem. 23: 11 (1988), discloses discovery of ligands for the mu-opioid receptor by this approach; as does Romar et al., Nature 298: 760 (1982), for the kappa-opioid receptor.
Screening of complex mixtures of natural products has generally been more successful, especially for the discovery of peptidase inhibitors. For example, Ferreira et al., Biochemistry 9: 2583 (1970), discloses the discovery of the ACE inhibitor, teprotide, by screening the venom of Bothrops iaraca. This approach may also be applied to the discovery of receptor ligands. Chang et al., Science 230: 177 (1985), discloses the discovery of the CCK antagonist asperlicin, using this approach.
Specific design of peptide mimetics has utilized both peptide backbone modifications and chemical mimics of peptide secondary structure. Spatola, Chemistry and Biochemistry of Amino Acids, Peptides and Proteins, Vol. VII (Weinstein, Ed.) Marcel Dekker, New York (1983), p. 267, exhaustively reviews isosteric amide bond mimics which have been introduced into biologically active peptides. The beta-turn has been implicated as an important site for molecular recognition in many biologically active peptides. Consequently, peptides containing conformationally constrained mimetics of beta-turns are particularly desirable. Such peptides have been produced using either external or internal beta-turn mimetics.
External beta-turn mimetics were the first to be produced. Friedinger et al., Science 210: 656-658 (1980), discloses a conformationally constrained nonpeptide beta-turn mimetic monocyclic lactam that can readily be substituted into peptide sequences via its amino and carboxy termini, and that when substituted for Gly.sup.6 -Leu.sup.7 in luteinizing hormone releasing hormone (LHRH), produces a potent agonist of LHRH activity.
Monocyclic lactams have generally been useful as external beta-turn mimetics for studying receptor-peptide interactions. However, the mimetic skeleton in these molecules is external to the beta-turn, which gives rise to numerous limitations. Chief among these is bulkiness, which requires the use of dipeptide mimetics, rather than mimetics of all four residues in an actual beta-turn. Substantial flexibility retained in these beta-turn mimetics makes it unsafe to assume that expected conformations are present, absent considerable conformational analysis. For example, Vallee et al., Int. J. Pept. Prot. Res. 33: 181-190 (1989), discloses that a monocyclic lactam beta-turn mimetic did not contain an expected type II' beta-turn in its crystal structure. Another limitation of the monocyclic lactam beta-turn mimetics arises from the difficulty of producing molecules that effectively mimic the side chains of the natural peptide. These difficulties arise from steric hindrance by the mimetic skeleton, which results in a more effective mimic of the peptide backbone than of the side chains. Considering the great importance of side chains in receptor binding, these difficulties strongly limit the versatility of monocyclic lactams.
Although the use of bicyclic lactams reduces problems of flexibility somewhat, conformational analysis of peptides containing these mimetics may still be desirable. Moreover, the side chain hindrance in these molecules may be even worse than that in the monocyclic lactams. Finally, both monocyclic and bicyclic lactams mimic only type II and type II' beta-turns, whereas type I and type III beta-turns are more prevalent in proteins and presumably in peptides.
The limitations presented by external beta-turn mimetics may be minimized by using mimetics in which the mimetic skeleton approximately replaces the space that was occupied by the peptide backbone in the natural beta-turn. Such molecules are known as internal beta-turn mimetics. Internal beta-turn mimetics may not generally reproduce the geometry of the peptide backbone of the particular beta-turn as accurately as external beta-turn mimetics. However, the internal position of the constraint allows replacement of larger sections of peptide, thus making tetrapeptide mimetics possible. The lack of bulk also diminishes the likelihood of steric hindrance of the side chains by the mimetic skeleton.
Internal beta-turn mimetics having biological activity are known in the art. For example, Krstenasky et al., Biochem. Biophys. Commun. 109: 1368-1374 (1982), discloses a leucine enkephalin analog in which an internal beta-turn mimetic replaced the residues Gly.sup.2 -Gly.sup.3 -Phe.sup.4 -Leu.sup.5, and which acted as an analgesic with one-third the potency of morphine. Other internal beta-turn mimetics have been described.
Kahn et al., Tetrahedron Lett. 27: 4841-4844 (1986), discloses an internal beta-turn mimetic, based upon an indolizidinone skeleton, and designed to mimic the lysine and arginine side-chain disposition of the immunosuppressing tripeptide Lys-Pro-Arg.
Kahn et al., Heterocycles 25: 29-31 (1987), discloses an internal beta-turn mimetic, based upon an indolizidinone skeleton, and designed to correctly position the aspartyl and arginyl side chains of a beta-turn in the proposed bioactive region of erabutoxin.
Kahn et al., Tetrahedron Lett. 28: 1623-1626 (1987), discloses a type I beta-turn mimetic which can be incorporated into a peptide via its amino and carboxy termini, and which is designed to mimic an idealized type I beta-turn. See also Kahn et al., J. Am. Chem. Soc. 110: 1638-1639 (1988); Kahn et al., J. Mol. Recogn. 1: 75-79 (1988).
Similarly, Kemp et al., Tetrahedron Lett. 29: 5057-5060 (1988), discloses a type II beta-turn mimetic which can be incorporated into a peptide via its amino and carboxy termini.
Arrhemius et al., Proc. Am. Peptide Syrup., Rivier and Marshall, Eds., Escom, Leiden (1990), discloses substitution of an amide-amide backbone hydrogen bond with a covalent hydrogen bond mimic to produce an alpha-helix mimetic.
Thus, there have been numerous successes in obtaining mimetics which can force or stabilize peptide secondary structure. However, little success has been reported in incorporating mimetics at the active site of a peptide hormone or neurotransmitter, probably because of the difficulty of producing mimetics that possess appropriately positioned side chain groups. There is therefore, a need for improved mimetics having greater substituent flexibility to allow for easy synthesis of mimetics having appropriately positioned side chain groups. Moreover, there is a need for improved mimetics having more readily controllable skeletal sizes and angles, so that different types of beta-turn structures can be easily imitated. An ideal mimetic would provide ready control and variation of both side chain positioning and mimetic skeleton size and angles through a modular construction system that allows easy synthesis of a wide variety of mimetics.
For recent reviews of the related art, see Hruby et al., Biochem. J. 2.68: 249-262 (1990); Ball et al., J. Mol. Recogn. 3: 55-64 (1990); Morgan et al., Ann. Rep. Meal. Chem. 24: 243-252 (1989); and Fauchere, Adv. Drug Res. 15: 29-69 (1986).